Viral infection of humans is a major health problem. Combating viral infection has proven to be highly effective in some cases like smallpox where the disease was essentially eradicated with the advent of the smallpox vaccination. Other viral infections have been much more difficult to fight. Hepatitis B and C, human immunodeficiency virus (HIV), and herpes viruses are just a few prominent members of a list of viruses that pose significant health threats worldwide. Treatments available for these viruses are associated with adverse side-effects. In some cases the viruses mutate and become resistant to a particular treatment. Accordingly, there is a clear need for new antiviral treatments.
Hepatitis virus B (HBV) is estimated to be responsible for approximately 4000-5000 deaths each year in the United States alone. The problem is substantially greater in the rest of the world and accounts for about 1 million deaths worldwide each year.
According to the Center for Disease Control, about one in twenty people in the United States will get infected with HBV. About two billion people worldwide have been infected with HBV, and 350 million of those have chronic infection. It is estimated that upwards of 10% of the population of the developing world (large parts of Asia, the Pacific, and sub-Saharan Africa) are chronically infected with HBV. Chronically infected individuals are at a much greater risk for hepatocellular carcinoma (liver cancer) and cirrhosis of the liver. Liver cancer caused by HBV is one of the leading causes of death in men by cancer in the developing world. By any analysis, HBV is one of the more serious health problems facing the world. HBV is transmitted by contact with blood or other body fluids of an infected person and is similar to the human immunodeficiency virus in that sense, but it is about two orders of magnitude more infectious than HIV. The main paths of transmission are from mother to infant at birth, child-to-child, unsafe injections and transfusions, and unsafe sexual contact.
HBV was probably first reported in the late 19th century in shipyard workers in Bremen who had been vaccinated against smallpox. Over the next one hundred years the association of hepatitis with the use of needles and syringes used for treating diseases like syphilis, diabetes, and the administration of vaccines for yellow fever, led scientists to discover HBV. Eventually a vaccine was developed from a viral envelope protein that was purified from the plasma of individuals with a chronic HBV infection. Later, a recombinant system was used to produce the protein used in the vaccines because the original source of the vaccine was those individuals infected with HBV and this same population was at high-risk for HIV infection.
The HBV vaccine was shown to be about 95% effective in preventing chronic infection in individuals if they were not previously infected. Unfortunately, the HBV vaccine does not cure the disease and there is a need from new treatments.
HBV is a circular DNA virus with double stranded and single stranded DNA with a genome of about 2300 nucleotides long for the full length strand and from about 1700-2800 nucleotides long for the short length strand.
After cell entry via attachment of HBV to host receptors and endocytosis, relaxed circular HBV DNA is transported to the nucleus where it is “repaired” to form covalently closed circular DNA (cccDNA). HBV DNA is replicated through an RNA intermediate.
This discovery now provides an entirely new class of HBV antivirals useful for treating and preventing HBV infection and related diseases like viral hepatitis and liver cancer.
One long standing difficult goal in antiviral research has been the search of host cell factors that can be target for treating and preventing viral infection.
A group of enzymes known as lysine methyl transferases and lysine demethylases are involved histone lysine modifications. One particular human lysine demethylase enzyme called Lysine Specific Demethylase-1 (LSD1) was recently discovered (Shi et al. (2004) Cell 119:941) and shown to be involved in histone lysine methylation. LSD1 has a fair degree of structural similarity, and amino acid identity/homology to polyamine oxidases and monoamine oxidases, all of which (i.e., MAO-A, MAO-B and LSD1) are flavin dependent amine oxidases which catalyze the oxidation of nitrogen-hydrogen bonds and/or nitrogen carbon bonds. Although the main target of LSD1 appears to be mono- and di-methylated histone lysines, specifically H3K4 and H3K9, there is evidence in the literature that LSD1 can demethylate methylated lysines on non-histone proteins like p53, E2F1, Dnmt1 and STAT3.
Several groups have reported LSD1 inhibitors in the literature. Sharma et al. recently reported a new series of urea and thiourea analogs based on an earlier series of polyamines which were shown to inhibit LSD1 and modulate histone methylation and gene expression in cells (J. Med. Chem. 2010 PMID: 20568780 [PubMed—as supplied by publisher]). Sharma et al. note that “To date, only a few existing compounds have been shown to inhibit LSD1.” Some efforts were made to make analogs of the histone peptide that is methylated by the enzyme, other efforts have focused on more small molecule like molecules based on known MAO inhibitors. Gooden et al. reported trans-2-arylcyclopropylamine analogues that inhibit LSD1 with Ki values is the range of 188-566 micromolar (Gooden et al. ((2008) Bioorg. Med. Chem. Let. 18:3047-3051)). Most of these compounds were more potent against MAO-A as compared to MAO-B. Ueda et al. ((2009) J. Am. Chem. Soc. 131 (48): 17536-17537) reported cyclopropylamine analogs selective for LSD1 over MAO-A and MAO-B that were designed based on reported X-ray crystal structures of these enzymes with a phenylcyclopropylamine-FAD adduct and a FAD-N-propargyl lysine peptide. The reported IC50 values for phenylcyclopropylamine were about 32 micromolar for LSD1 whereas as compounds 1 and 2 had values of 2.5 and 1.9 micromolar respectively.
Importantly, studies have also been conducted on amine oxidase inhibitor compounds to determine selectivity for MAO-A versus MAO-B since MAO-A inhibitors can cause dangerous side-effects (see e.g., Yoshida et al. (2004) Bioorg. Med. Chem. 12(10):2645-2652; Hruschka et al. (2008) Biorg Med. Chem. (16):7148-7166; Folks et al. (1983) J. Clin. Psychopharmacol. (3)249; and Youdim et al. (1983) Mod. Probl. Pharmacopsychiatry (19):63).
Currently the treatments available for HBV and related diseases have serious drawbacks. There is a need for new drugs for these diseases that target novel points of intervention in the disease processes and avoid side-effects associated with certain targets.